The present invention relates to a process for the manufacture of copolyamides of terephthalic acid, and especially to the manufacture of such copolyamides without formation of a high melting fraction.
As used herein, a "single step" polymerization process is a polymerization process in which monomers are subjected to a polymerization cycle in a reactor that concludes with the pressure being returned to substantially atmospheric pressure. It is understood that the copolyamide obtained from such a process may be subjected to further processing, including finishing and extrusion, before or after pelletization.
Polymers formed from terephthalic acid and diamines are known in the art. For instance, PCT patent application CA91/00442 of S. L. Mok and R. U. Pagilagan, filed 11 Dec. 1991, discloses copolyamides of terephthalic acid with hexamethylene diamine and 2-methyl pentamethylene diamine, optionally containing isophthalic acid, having melting points of at least 280.degree. C. Other polyamides of terephthalic acid are referred to therein.
Single-step polymerization processes have the advantage that polymer is manufactured in a single step, as opposed to two step or multi-step processes, thereby reducing handling of the partially formed polymer during the intermediate stages of the process. However, in the manufacture of copolyamides of terephthalic acid, especially copolyamides of terephthalic acid, hexamethylene diamine and 2-methyl pentamethylene diamine using a single step polymerization process, it has been found that a high melting fraction is formed.
The high melting fraction, with a melting point in excess of 320.degree. C., and especially approximately 334.degree. C., is usually not noticed in the first batch of polymer produced in an autoclave, but becomes apparent in subsequent batches made in the same autoclave without steps being taken to clean-out the autoclave between consecutive batches. As the polymer would normally be subsequently processed at melt temperatures of less than 320.degree. C., the high melting fraction manifests itself as unmelted or gel-like portions of polymer, which are commercially unacceptable; the gel-like portions are believed to be essentially comprised of a high melting crystalline phase, which is soluble in sulphuric acid, and not gel per se which would not be soluble in sulphuric acid.
It has now been found that copolymers of terephthalic acid, optionally including isophthalic acid, and mixtures of hexamethylene diamine and 2-methyl pentamethylene diamine, may be manufactured without the formation of a high melting fraction in significant amounts.